The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for designing, optimizing, and shimming multiband radio frequency (“RF”) pulses for use in MRI.
The recent availability of high and ultrahigh magnetic fields in MRI, such as fields of 3 Tesla (3T) and 7 Tesla (7T), has enabled anatomical and functional MRI (“fMRI”) studies of the human brain with increasingly higher spatial resolution and, in the case of fMRI applications, fidelity to sites of elevated neuronal activity. Obtaining such high-resolution fMRI data over the entire human brain, however, encounters the undesirable consequence of long volume repetition times (“TRs”) even when single-shot slice acquisition methods, such as echo-planar imaging (“EPI”), are employed. Simultaneous multiband (“MB”) RF excitation of multiple slices with subsequent unaliasing of the simultaneously acquired slices through parallel imaging principles using a multi-slice, two-dimensional EPI strategy has been used to address these problems. This method allows for a direct reduction in the volume TR by a factor (termed the MB factor) that is equal to the number of simultaneously excited slices.
The MB approach has been employed with significant success in task-based and in resting-state fMRI, leading to improved detection of resting state networks and new analysis strategies that reveal the temporal dynamics of such networks. The approach has also been employed to significantly reduce the otherwise very long imaging acquisition time in diffusion imaging techniques, such as high angular diffusion weighted imaging (“HARDI”) and diffusion spectrum imaging (“DSI”). As such, the use of MB RF pulses has made the use of these imaging techniques practical and indispensable for efforts like the Human Connectome Project. In these applications, the achievable MB factors tolerated by the unaliasing process can be significantly improved through partial shifting of simultaneously acquired slices along the phase encode dimension using gradient blips in EPI. It is noted that the MB approach can be used with spatial encoding strategies other than EPI, such as gradient-recalled echoes acquired one k-space line at a time (e.g., FLASH), fast spin echo, and so on. In addition, the MB approach is applicable to imaging applications other than fMRI, including generally any imaging application in which multiple slices are acquired substantially simultaneously.
Despite these gains, however, the optimal use of the slice accelerated multiband approach at high (3T and 4T) and ultrahigh (7T and higher) fields is precluded by transmit B1 (“B1+”) inhomogeneities and power deposition constraints. Signal-to-noise ratio (“SNR”) and image contrast becomes spatially non-uniform and, at some locations, suboptimal due to non-uniform B1+ that arises as a result of the destructive interferences generated in the presence of the traveling wave behavior of RF at these field strengths. These B1+ inhomogeneities have been well documented at 4T and particularly at 7T, but are also sufficiently strong at 3T to cause SNR differences between the central and the peripheral parts of the brain, especially in a spin-echo (“SE”) based sequence. Similarly, maximal achievable slice acceleration factors can be limited by power deposition especially at ultrahigh fields and/or when SE sequences are employed. When the number of slices and the volume TR are kept the same, the multiband approach does not deposit any more power compared to the conventional single slice excitation, even though peak power may increase quadratically with the MB factor; however, accelerating by the MB factor leads to MB-fold increase in power deposition, which imposes a limit on achievable accelerations.
These limitations are also paramount in imaging of the human torso and extremities in clinical diagnosis. In particular, imaging using sequences such as turbo spin echo, also known as fast spin echo and related derivatives, which are the basis of a large number of clinical scans in brain, torso, and extremities imaging, would suffer from power deposition and B1+ inhomogeneities when implemented with the multiband approach.
It would therefore be desirable to provide a method for designing and providing multiband RF pulses for multichannel transmission applications, in which the designed multiband RF pulses have reduced B1+ inhomogeneities and lowered power deposition (including lowered global SAR, lowered peak local SAR, and/or lowered peak RF power) as compared to currently available multichannel, multiband RF techniques.